1. Field of the Invention
The present invention relates to an immersion nozzle for introducing molten metal from a tundish to a mold for continuous casting of molten metal, and more particularly to a structure of the immersion nozzle.
2. Description of the Related Arts
An immersion nozzle made of refractory material is used to introduce molten steel from a tundish to a continuous casting mold. Particularly when a high speed casting for producing a slab is carried out, a shape of an immersion nozzle which has a pair of exit ports opening toward a shorter side of the mold as shown in FIG. 20 (A) and FIG. 20 (B) presently is used in general.
And in a continuous casting, it is generally required to solidly the molten steel stably and to remove, by floatation, non-metallic inclusion contained in the molten steel which causes ingot defects.
Therefore, an immersion nozzle is required to have flow of molten steel dispersed uniformly in the mold, to have non-metallic inclusion floated, and to give an adequately uniform flow of the molten steel on the surface of the molten steel in the mold. In addition, it is wanted that the molten steel which is discharged from left and right exit ports on a side wall of the nozzle and then moves toward the shorter side on left and right of the mold makes no flow difference on both directions and that the molten steel flow which hits the shorter side of the mold and is separated into an upward flow and a downward flow gives an adequate flow rate in a shorter side upward flow.
If a surface flow speed of the molten steel within the mold is not in an adequate range, problems described below occur. When the surface flow speed of the molten steel is lower than the adequate range, the heat of the molten steel introduced through the exit ports is insufficient, which may cause a partial solidification of the molten steel surface to bring the solidified pieces into an ingot to result in an ingot defect and, in the worst case, in an interruption of casting operation. When the surface flow speed of the molten steel is above the adequate range or when an unbalance flow of the molten steel is excessive, powder floating on the molten steel surface is entrapped into the ingot to raise powder defects, which causes degradation of the ingot in quality.
When the unbalance flow of the molten steel occurs, a penetration depth of the molten steel flow downward into a molten steel pool in the mold increases by 20 to 40% more than that of normal operation, which makes floatation of alumina inclusion difficult.
To solve the problems described above, an immersion nozzle as illustrated in FIG. 21 was disclosed by Unexamined Japanese Patent Publication No. 62-296944. The disclosed immersion nozzle has a pair of exit ports opened at a side wall body of the nozzle toward the shorter side wall of the mold with a downward slope and a slit opened at the bottom of the nozzle, crossing the nozzle bottom with an angled shape while connecting with both exit ports (this type of nozzle is hereinafter referred to as "a two-exit port nozzle with connected slit").
Another type of immersion nozzle was disclosed in Unexamined Japanese Patent Publication No. 61-14051, which is shown in FIG. 22. The nozzle is also a two-exit port nozzle with connected slit which connects left and right two-exit port on the side wall of the nozzle with the slit crossing the tip of the nozzle. In that case, however, the shape of the nozzle tip is hemispherical.
Since that type of immersion nozzle feeds a part of the molten steel downward into the mold through the slit opened at the tip of the nozzle, the quantity of the molten steel fed through the side exit ports toward the shorter sides of the mold decreases, and the surface flow speed of the molten steel in the mold reduces, which prevents inclusion of mold powder on the molten steel surface.
FIG. 23 illustrates a water model experimental result simulating a molten steel flow pattern inside of the mold using a two-exit port nozzle with connected slit of FIG. 21. The molten steel flows out through the exit ports of the side wall of the nozzle and moves toward the shorter side of the mold, hits the solidification shell of ingot, and is separated into an upward flow along the shorter side (hereinafter referred to as "short side upflow") and a downward flow (hereinafter referred to as "short side downflow"). The shorter side upflow reaches the molten steel surface in the mold to swell up at the surface of the molten steel, then it becomes a surface flow moving from the shorter side toward the center of the mold.
Since the nozzle feeds the molten steel also through the bottom slit, the speed of the short side upflow is small, and the fluctuation of the surface on the molten steel in the mold is also small. In addition, the molten steel which has flowed out through the bottom slit spreads in a width direction of the mold to result in a shallow penetration depth into the molten steel. The nozzle of FIG. 21, however, induces an unbalanced flow where a flow rate of the molten steel through one side outlet of the exit ports increases, while another flow rate coming through another side outlet of the exit ports decreases. Consequently, on the outlet side of the excessive discharge, the short side upflow is enhanced to increase the fluctuation of the surface of the molten steel.
Furthermore, the molten steel flowing out through the bottom slit does not spread in the width direction of the mold, and a flow band segregates to the side of enhanced short side upflow. Accordingly, the flow band competes with the short side downflow to generate a powerful downflow (hereinafter referred to as "mold downflow") which penetrates deep into the mold.
The unbalance of the molten steel flowing out through the exit ports differs, in the left or the right side of the mold as time passes. As a result, there occurs an abnormal surface level fluctuation of the molten steel in the mold, generating vortices in the vicinity of the nozzle to induce inclusion of mold powder. Further, no improvement in penetration depth of non-metallic inclusion in the molten steel is made due to the mold downflow, either. Thus, that type of nozzle gives very few improvements compared with a prior art nozzle of two-exit ports which does not have the bottom slit (FIG. 20(a) and FIG. 20(b)).